Variability of the Observed Midlatitude Storm Tracks in Relation to Low-Frequency Changes in the Circulation Pattern

Abstract

The principal modes of month-to-month variability of the wintertime storm tracks over the North Pacific and North Atlantic are identified by empirical orthogonal function analysis of the root-mean-square statistics of bandpass (2.5?6 day) filtered geopotential height data for 19 yr. One of the two leading modes depicts fluctuations in the level of synoptic-scale activity without any noticeable spatial displacement of the storm track axes, whereas the other mode is associated with meridional shifts of the storm tracks from their time-averaged positions. Higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions. It is demonstrated that the leading storm track modes are linked to some of the best-known monthly averaged teleconnection patterns. The dipolar western Pacific and western Atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans, whereas the more wave-like Pacific/North American and eastern Atlantic teleconnection patterns are coincident with north?south displacements of the storm track axes over the eastern oceans. The representative synoptic scenarios for various storm track modes are portrayed using composite charts. These patterns illustrate the strong modulation of the trajectory of weather systems by the intensity and steering action of the monthly averaged flow field, so that the storm tracks are preferentially located at and slightly downstream of the quasi-stationary troughs. The shape and propagation of the synoptic scale eddies along the changing storm tracks, as well as the barotropic interactions between these disturbances and the monthly mew flow, are diagnosed using composite patterns of extended Eliassen-Palm vectors and eddy-induced geopotential tendencies at 300 mb. It is seen that the synoptic-scale fluctuations are typically crescent-shaped, and sometimes undergo noticeable deformation when they encounter quasi-stationary ridges. In the upper troposphere, enhanced eddy activity is accompanied locally by eastward acceleration, as well as by positive geopotential tendency immediately to the south, and negative geopotential tendency to the north, and vice versa. The distributions of eddy-induced geopotential tendency for individual storm track modes indicate a near inphase relationship between the synoptic scale barotropic forcing and the quasi-stationary flow pattern at 300 mb. The characteristic time scale for this forcing is approximately 7?10 days. The characteristic circulations at sea level associated with various storm track modes are examined using composite charts of the sea level pressure field. Some of these composites resemble the patterns associated the North Pacific and North Atlantic Oscillations.